The resin transfer molding (RTM) method is a known method that is used for making parts out of (fiber-resin) composite materials having a high strength to weight ratio. In conventional manner, the RTM method comprises the following operations:                preparing a fiber preform by weaving fibers;        placing the fiber preform in an injection mold;        injecting resin in the liquid state into the mold in order to fill it and impregnate the fiber preform; and        applying pressure to the injected resin and polymerizing the resin by heating.        
Such a method is known, e.g. from document EP 0 068 512.
After unmolding, a composite part is obtained that comprises a fiber reinforcing structure embedded in a matrix of polymerized resin. The outside shape of the part corresponds to the shape taken by the resin in the cavity of the mold.
The resins used are very fluid so as to penetrate well between the various fibers of the preform, even when injected under low pressure. During polymerization, under the effect of heat, the injected resin passes successively from the liquid state to the gelled state to the solid state.
In order to avoid or limit defects and pores in the part, associated with the phenomenon of the resin giving off gas during polymerization, it is known to maintain the resin under pressure until the part has polymerized completely. For this purpose, it is possible to use a pressurizing device such as for example the device described in patent Document WO 2013/068666 A1. Although that solution is satisfactory for defects and pores of medium size, it can be found difficult to perform for defects and pores of small size (e.g. of size smaller than 5 millimeters (mm)). Specifically, under such circumstances, the level of pressure required for the resin often exceeds the capacity of devices in common use, which makes it necessary to use other devices that are more expensive and to adapt existing installations to those new devices.
There therefore exists a need for a new solution.